1. Field of the Invention
This invention relates to an inkjet ink, an inkjet recording method, a method of evaluating an inkjet ink, and a method of manufacturing the inkjet ink.
2. Description of the Related Art
In recent years, there has been increasing demands for an inkjet type printer. Namely, it is expected that the inkjet type printer which is high in printing speed and capable of achieving high-quality printing is useful not only as a printer for personal computers in offices and at home but also as a printer for printing a fairly large number of copies of printed matter such as a poster of large scale, a local advertisement bill and an enterprise handout, thus substituting for the conventional printing utilizing a form plate. This inkjet type printer may be considered as being an on-demand printer which is capable of quickly coping with the diversification of needs.
As for the ink to be employed in the inkjet type printer, an aqueous ink using water-soluble dyes has been mainly employed in viewpoint of overcoming the problems of odor and safety. In recent years however, inks employing pigments as a colorant have been proposed in order to improve the quality of printing such as water resistance and light stability. To meet such a proposal, it is now required to control the particle diameter of pigments to be included in the ink and to secure the stability of dispersed state of pigments. If the dispersed state of pigments is instable, the flocculation of pigment particles will occur. In the case of inkjet type printer in particular, the flocculation of pigment particles may trigger the instability in the delivering state of ink. As a result, the rectlinearity in delivery of ink would be degraded, thus generating streak lines and the like on printed matters and hence making it impossible to obtain printing of high quality. In the worst case, there may be a case where the delivery port is clogged on the occasion of delivering ink. There is also the problem that the ink would become poor in long-term storage stability.
In order to solve the aforementioned problems, it is required to control the state of pigment particles. Up to date however, no one has succeeded to find means to directly observe the particles of pigment as well as the flocculation of pigment particles. As for the means conventionally available to obtain the information with regard to the particle diameter of pigments, etc., there have been employed various kinds of particle size-measuring apparatuses where a dynamic light scattering method, a light diffraction scattering method or a laser Doppler method are utilized. However, even with these measuring apparatuses, the particle size to be obtained from these measuring apparatuses is that of the population of the entire sample being measured, so that it has been very difficult to know the size of particle or the quantity of particle, especially that of flocculation which are contained only as a relatively small number. Further, under some circumstances, it is required to dilute a measuring sample on the occasion of measuring the particle size. However, there are possibilities that the state of dispersion of pigment particles when the measuring sample is diluted is not the same as the state of dispersion of pigment particles when the pigment particles exist as they are in an actual ink. It is also tried to observe the state of pigment particles by electronic microscope. In this case however, the measurement is directed not to the state of pigment particles in the ink but to the simple substance of pigment itself. Therefore, there are possibilities that the state of dispersion of pigment particles as measured by electronic microscope is not the same as the state of dispersion of pigment particles when the pigment particles exist as they are in an actual ink. Because of this, it has been very difficult to know not only the particle size of pigment particles and of the aggregate of pigment particles but also the distribution of the particle size thereof.
As for various kinds of ink that have been disclosed up to date, there are known an ink wherein only the range of average particle diameter is regulated, an ink wherein the distributed ratio of pigment particles included in the distribution range is regulated, an ink wherein the volume ratio of distributed pigment particles having a particle diameter of 1 μm or more is regulated, etc. These inks are all formed of an aqueous ink, and, moreover, it is considered difficult to considerably enhance the delivery stability of ink by simply controlling only the average particle diameter of pigment particles or the distribution thereof. Furthermore, even if only the dispersed pigment particles having a particle diameter of 1 μm or more is regulated, it is considered impossible to considerably enhance the delivery stability of ink. Namely, even if only the average particle diameter of pigment particles or the distribution thereof is regulated, it does not lead to the reduction in a sufficient degree of the number of coarse pigment particles. Further, even if it is possible to regulate the volume ratio of the dispersed pigment particles having a diameter of 1 μm or more, it is impossible, according to the conventional methods, to detect the aggregate (secondary particle) of dispersed pigment particles having a diameter of less than 1 μm as well as the flocculate formed of a plurality of secondary particles which are weakly bonded together. Because of these reasons, it has been impossible to sufficiently eliminate the flocculate of pigment particles and to considerably enhance the ink delivery stability.